1. Field of the Invention
This invention pertains to capturing X-ray images and, more particularly, to a filmless, self-contained, portable electronic cassette and associated process for capturing and recording digital representations of radiographic images.
2. Description of the Related Art
Well known medical diagnostic processes record X-ray images on silver halide-based films. These systems generally require directing a beam of X-radiation through the subject to be studied, intercepting the beam with an image intensifying element, recording the intensified and imagewise modulated beam in a silver halide based film, and chemically transforming this captured latent image into a permanent and visible image, called a radiogram. See U.S. Pat. No. 5,070,248, titled CASSETTE WITH INTENSIFYING SCREENS TO BE USED WITH AN X-RAY FILM. Significant progress has been made in this area by increasing the effectiveness of the intensifying screen to reduce patient X-ray exposure levels, packaging the system so it can be used in daylight conditions, and creating digitized representations of the image that can be manipulated and interpolated in a multitude of ways to aid in interpretation of the recorded image. It would be desirable to eliminate the expense and time consumed by use of film sheets and chemical processing.
Progress has been made in introducing systems that do not on traditional silver halide-based films. The radiography process captures a latent radiographic image using a photoconductive layer sensitive to X-radiation (L. Jeromin, "Electroradiography", in Encyclopedia of Medical and Instrumentation, edited by J G Webster, Wiley, New York, 1988). Before exposure to X-radiation, the surface of the photoconductive layer is uniformly charged; after exposure to X-radiation, depending on the intensity of the imagewise modulated radiation, electron-hole pairs generated by the X-radiation are separated by an electric field incident to the charges laid over the surface and move along the field to recombine with the surface charge. After X-ray exposure, a latent image in the form of electrical charges of varying magnitude remain on the plate surface and may be rendered visible by toning and transferring the image to a receiving sheet.
Efforts to eliminate the recording film also include using a layer of stimulable phosphor to intercept the radiation transmitted image. See, for example, U.S. Pat. No. 4,931,643, AUTORADIOGRAPHY SYSTEM FOR STIMULABLE PHOSPHOR FOILS. The phosphor is chosen such that it emits light corresponding to the latent image stored therein when subsequently scanned with stimulating rays. Such systems generally employ a recording apparatus for storing the image in the stimulable phosphor, a radiation image read-out apparatus for reading out the image stored on the stimulable phosphor, and frequently, image reproducing apparatus. These are usually provided independently of one another, so that size and manufacturing costs remain large. One improvement on this approach is described in U.S. Pat. No. 4,975,580, RADIATION IMAGE READ-OUT AND REPRODUCING APPARATUS, wherein the image read-out and reproduction are carried out by use of the same scanning system. Further, means for conveying the stimulable phosphor sheet for read-out and for conveying the recording sheet for reproducing are common to one another. However, resolution of the image is degraded because of light-scattering in the storage phosphor.
These systems for using silver-halide based films, xeroradiography or stimulable phosphors suffer common problems since: (1) the image cannot be obtained immediately after exposure as time is consumed during transport of the cassette to the film processor and during subsequent film processing operations; (2) considerable time and equipment is required to load and unload un-reuseable films from cassettes and to develop films into radiograms; and, (3) the medically valuable information is not in a format that can readily provide a digital output if image processing is used to enhance diagnostic analysis. It is therefore desirable to be able to directly record radiograms in a digital format, without further processing shortly after exposure, and to also eliminate the equipment used for loading, unloading, and developing traditional X-ray film based systems and reuseable phosphor screen systems.
Efforts to overcome these deficiencies include the use of an electrostatic panel comprising a photoconductive layer over an insulating layer on a conductive support, the photoconductive layer also covered by a dielectric layer, the dielectric layer overcoated with a plurality of microplates that form a microcapacitor structure. Improvements in this method, involving the use of a plurality of thin-film transistors interconnected with a network of conductive lines orthogonal to one another to further increase the resolution, have been described in a co-pending application, Docket No. IM-0817, also assigned to Du Pont.